In drilling of wells, drilling fluid is generally circulated through a drill string and drill bit and then back to the surface of the wellbore being drilled. At the surface, the fluid is processed to remove cuttings and to maintain desired properties before it is recirculated back to the well. During drilling operations, some amount of this drilling fluid may be lost due to various factors. This loss of drilling fluid may be referred to as lost circulation. Lost circulation is one of the largest contributors to non-productive time in drilling operations. This is particularly true for wells being drilled in complex geological settings such as deep water or highly depleted zones or intervals. Thus, it is important to determine the causes of lost circulation and try to mitigate those factors.
One major factor that has been identified to cause lost circulation is the formation of fractures in the wellbore wall. These fractures provide an outlet for the drilling fluid to escape from and thus result in loss of fluids. Losses caused by factures are particularly troublesome, as they can be uncontrollable in large volumes. To prevent or mitigate wellbore losses, an engineering practice referred to as wellbore strengthening may be conducted to increase the pressure at which a fracture will form in the wellbore wall, known as fracture gradient (FG), or to prevent already created fracture(s) from further propagation.
Wellbore strengthening involves sealing existing natural fractures or induced fractures with materials having properties that are conducive to sealing of the wellbore wall to mitigate further fracture propagation. In general, to conduct a successful wellbore strengthening operation, width of a fracture at the wellbore wall (i.e. fracture width profile) has to be determined. This allows accurately engineering lost circulation material to have a suitable particle size distribution that can seal the fracture at the wellbore wall.
Conventional wellbore strengthening applications generally involve optimizing drilling fluid particle size distribution to seal fractures created during drilling operation. However, wellbore strengthening may also involve creating intentionally induced fractures that are then sealed. This has been shown to mitigate initiation and propagation of new fractures around the wellbore. To create intentionally induced fractures, mud weight can be used to exert extra pressure on the formation. When pressure exerted by mud weight exceeds FG of the wellbore at a particular point in the well, a fracture is created at that point.
However, because of difficulties associated with having a precise mud weight at particular locations in the well and because of uncertainties associated with drilling operations, it is difficult to control the accuracy of the process. Imprecise pressure at the wellbore wall might cause uncontrollable growth of induced fractures. This can result in fractures that have unacceptably larger widths and/or ones that extend too long into the formation. The following disclosure addresses these and other issues.